Processes for the conversion of lower molecular weight alkanes such as methane to higher molecular weight hydrocarbons which have greater value are sought. One of the proposals for the conversion of lower molecular weight alkanes is by oxidative coupling. For instance, G. E. Keller and M. M. Bhasin disclose in Journal of Catalysis, Volume 73, pages 9 to 19 (1982) that methane can be converted to, e.g., ethylene. The publication by Keller, et al., has preceded the advent of substantial patent and open literature disclosures by numerous researchers pertaining to processes for the oxidative coupling of lower alkanes and catalysts for such processes.
In order for an oxidative coupling process to be commercially attractive, the process should be capable of providing a good rate of conversion of the lower alkanes with high selectivity to the sought higher molecular weight hydrocarbons. Since conversion and selectivity can be enhanced by catalysts, catalytic processes have been the thrust of work done by researchers in oxidative coupling. However, not only are conversion and selectivity sought, but also, the catalyst must have sufficient stability (or useful life) to be commercially attractive.
Recently, Choudhary, et al., in "Oxidative Coupling of Methane to C.sub.2 -Hydrocarbons over La-promoted MgO Catalysts", J. Chem. Soc., Chem. Commun., 555 (1989) report:
"Earlier, a number of promoted MgO catalysts, viz. Li-MgO, Na-MgO, K-MgO, Na-Mn-MgO, PbO-MgO, and CaCl.sub.2 -MgO for oxidative coupling of methane to C.sub.2 -hydrocarbons have been reported. The promoters used earlier for enhancing both the activity and selectivity of MgO catalysts are low melting compounds and therefore during the catalytic process, the catalysts are expected to be deactivated owing to evaporation of active components and/or sintering of the catalysts." PA0 "The advantage of the definite compounds are the following: PA0 -the repartition and location of the added lithium is controlled in the starting materials; PA0 -the amount of added lithium is increased compared to the impregnation method; PA0 -as the lithium atoms are integrated in the crystal lattice, the loss of lithium due to its reacting with quartz apparatus should be limited." (L1)
The authors referenced their earlier work in Recent Trends in Chemical Engineering, eds. Kulkarni, et al., Wiley Eastern Ltd., New Delhi (1987) vol. I, p. 90. Also Korf, et al., "The Selective Oxidation of Methane to Ethane and Ethylene Over Doped and Un-doped Rare Earth Oxides," Catalysis Today, vol. 4, No. 3-4, 279 (Feb. 1989) disclose that Sm.sub.2 O.sub.3 catalyst, while initially providing a lower C.sub.2 (ethane and ethylene) yield than the lithium, calcium or sodium counterparts, after time on stream, each of the doped counterparts lose C.sub.2 yield. Moreover, after 50 hours on stream, the lithium and sodium doped counterparts yielded less C.sub.2 's than the undoped catalyst. Follmer, et al., in "The Application of Laboratory-Scale Catalytic Fixed and Fluidized Bed Reactors in the Oxidative Coupling of Methane", Symposium on Direct Conversion of Methane to Higher Homologues presented before the Division of Petroleum Chemistry, Inc., American Chemical Society, Los Angeles Meeting, Sept. 25-30, 453 (1988) also note that most alkali/alkaline earth compound catalysts deactivate with time-on-stream. The authors report a NaOH/CaO catalyst exhibiting better stability over 240 hours time of operation than a PbO/gamma-Al.sub.2 O.sub.3 catalyst. Changes in performance of the former catalyst were said to have been observed during only the first 10 hours of operation.
In many instances, the literature and patents describing oxidative coupling processes and catalysts do not relate experiences with catalyst stability. However, due to the lifetime problems that have plagued oxidative coupling catalysts, skepticism appears to exist that oxidative coupling catalysts exhibit long useful lifetimes absent empirical demonstrations.
Recent efforts have been directed toward avoiding the use of low melting temperature dopants which may tend to deactivate during this process. See, for instance, Choudhary, et al., supra. Kaddouri, et al., in "Oxidative Coupling of Methane over LnLiO.sub.2 Compounds (Ln=Sm, Nd, La)", Appl. Catal., vol. 51, L1 (1989) propose the use of "definite compounds" containing alkali metal, i.e., SmLiO.sub.2, LaLiO.sub.2 and NdLiO.sub.2 as oxidative coupling catalysts. The authors state:
While enhanced yield and selectivity to C.sub.2 's is reported by the authors in comparison to the non-alkali metal containing compounds, no data are provided on catalyst stability.
Perovskite (or perovskite-type) catalysts have been considered by several researchers. Perovskites are certain complex oxides having crystalline structures due to regular atom placement. See, for instance, Hazen, "Perovskites", Scientific American, June 1988, pages 74 to 81.
Otsuka, et al., Chem. Lett., 1835 (1987) report the oxidative coupling of methane using alkali or alkaline earth doped cerium oxide. They found a barium doped cerium oxide catalyst to provide a methane conversion of 40% at 750.degree. C. with a C.sub.2 selectivity of 37%. They believe that BaCeO.sub.3 is the catalytically active species. Similarly, Nagamoto, et al., Chem. Lett., 237 (1988) evaluated various ABO.sub.3 perovskites as methane coupling catalysts wherein calcium, strontium or barium is present in the A site and titanium, zirconium or cerium in the B site. The authors opine that catalytic activity is correlated with the basicity of the atom in the A position and deviations from the equilibrium value of the interatomic distances. Work on ABO.sub.3 perovskites is further reported by Shamsi, et al., Energy Progress, vol. 8, No. 4, 185 (1988) and Energy & Fuels, vol. 2, 235 (1988). These researchers substituted sodium or potassium into LaMnO.sub.3 and achieved improved performance. The authors postulate that the improvement is due to the formation of oxygen lattice defect sites that strongly bind oxygen species to a surface site. One catalyst, La.sub.0.9 Na.sub.0.1 MnO.sub.3 is said to have provided 21% methane conversion at 820.degree. C. with a 63% selectivity to C.sub.2 's. The substitution of gadolinium or samarium for the lanthanum is reported to provide comparable results.
In other complex oxides, Machida, et al., J. Chem. Soc., Chem. Commun. 1639 (1987), reported a SrCe.sub.0.9 Yb.sub.0.1 O.sub.2.95 catalyst which provided a methane conversion of 52.6% and C.sub.2 selectivity of 60% at 750.degree. C for methane coupling. The authors postulate that high oxygen ion conductance within the crystal lowers the C.sub.2 selectivity while high proton conductance within the crystal enhances it.
Rock salt structure catalysts, LiYO.sub.2 and LiNiO.sub.2, have been disclosed by Lambert, et al., Appl. Catal., vol. 42, L1 (1988).
Catalysts for the oxidative coupling of methane which contain lanthanide series oxides are disclosed in, for example, European Patent Application Publication No. 206044 (alkali metal doped La.sub.2 O.sub.3); U.S. Pat. No. 4,780,449; U.S. Pat. Nos. 4,499,323 and 4,499,324 (Ce or Pr containing alkali or alkaline earth metals); European Patent Application Publication No. 189079 (lithium doped samarium oxide); and Korf, et al., supra (oxides of Sm, Dy, Gd, La and Tb, with alkali and alkaline earth metal doping.)
Imai, et al., J. Catal., vol. 106, 394 (1987); J. Chem. Soc., Chem. Commun, 52 (1986) and React Kinet. Catal. Lett., vol. 37, 115 (1988) reported that amorphous lanthanum aluminum oxides are catalysts for the oxidative coupling of methane (e.g., methane conversion of 25% with C.sub.2 selectivity of 47%). The authors noted that the growth of crystallinity reduced catalyst activity. The activity and selectivitY provided by lanthanum aluminate is reported to be higher than those provided by the component oxides, i.e., alumina and lanthanum oxide. They preferred more amorphous materials.
J. Gopalakrishnan, et al., Inorg. Chem., vol. 26, 4299 (1987) have disclosed layered perovskites of the formula A.sub.2 Ln.sub.2 Ti.sub.3 O.sub.10 wherein A is potassium or rubidium and Ln is lanthanum, neodymium, samarium, gadolinium or dysprosium. These materials are said to exhibit ion exchange of the alkali metal in aqueous or molten salt media.